Little basic change has taken place in the principles of railroad freight car truck design since the late 1800's. The vehicles which ride on these trucks have changed radically, as have the vehicle load-carrying requirements and operating speeds. To meet the rapidly increasing demands for higher capacity and greater speed, truck designers merely scaled up. When a problem developed in one component, that component would be beefed up or some ancillary device would be added. Subsequently, problems were not solved but, rather, were chased from one component to another throughout the system.
Since elastic deformations of both rail and wheel, in contact with one another, are relatively small, rail joints, track gage and cross-level, and other rail conditions, all dynamically affect the wheels, axles and bearings as well as the dependent truck superstructure and suspension, carbody, and finally -- but most importantly -- the responsive lading in the car. The problems that the railroads are currently encountering in operation of freight cars with current track conditions and existing equipment are: rock-and-roll which is a resonant rolling mode induced by staggered rail joints being encountered alternately by the wheels on either side of the truck at low speed; high speed hunting which is an oscillation induced by the tendency of a wheel to recenter developing into a resonant lateral chatter during high speed operation; and the inability of standard truck design to control random motions which results in multiplication of forces developed during resonance.
A standard truck consists of 4 wheels, fixed in pairs on axles. The axles are fitted on the ends with roller bearings and are held substantially parallel by a pair of side frames. The side frames are fixed loosely by way of an adapter and a keeper to the roller bearing. Each side frame is a rigid truss member which spans between the axles and has an opening at its center called a spring pocket. This spring pocket contains a group of springs which support a bolster member. The bolster is a rigid transverse member which spans between the side frames, passing through the spring pockets, and is supported at its ends by the spring groups. At the center of the bolster is a center bowl. The center bowl supports the weight of the car by means of a mating centerplate which is affixed to the understructure of the car. The car is thereby supported on 2 points; one at each end of the car which gives a tendency to rock to one side or the other. The tendency of the car to rock is resisted by two side bearings, located on either side of the center bowl. Each side bearing is set with a clearance so that the carbody is not in contact unless it rocks to one side.
The axles are fixed to the wheels and the wheels, therefore, must rotate at the same speed. The wheels are tapered so that they tend to center themselves or so that when they ride up on a curve, the outer wheel can move to a larger radius allowing the wheels to remain traveling at the same speed. The bearings are press-fitted to the ends of the axles and are clamped into the end of the side frames by way of an adapter and keeper. The adapter is shaped so that the axle can rotate relative to the side frame in a horizontal plane without wracking or eccentrically loading the bearing. It is also set with a clearance so that it is allowed to move laterally in the opening. The keeper is not in contact with the bearing, but is placed so that the bearing cannot fall out. The relationship between the side frame and the bolster is approximately the same. The bolster is allowed to rotate relative to the side frame and to move laterally in the spring pocket and is engaged by a stop at the extreme position. These relative clearances and motions are allowed with the design purpose of preventing excessive twisting or wracking stresses in the bearing, the side frame, and the bolster end during operation. Unfortunately, to allow motion is to allow wear and to allow motion with the forces of the magnitude encountered by or developed by a railroad car, is to allow extremely high-impact forces and dynamic forces to develop.
The classes of devices presented as solutions to existing truck problems range from mechanical and hydraulic spring snubbers to constant contact side bearings and other truck-to-car body supportive arrangements. In every operating situation currently in the U.S., Canada and Mexico truck suspension elements include steel coil springs. Ancillary devices offered to control harmonic and other problems caused by use of steel coil springs include: steel volute springs (snubbers); hydraulic snubbers; steel friction snubbers. Each of these devices produces some special side effect that results in increased wear, maintenance, stress levels, fatigue, damage and cost. The same holds true for so-called constant-contact side bearings and truck bolster -- body bolster devices which directly affect the members to which they are affixed or contact as well as other, non-directly related parts of the entire system.
For example, a recent truck hunting investigation made by the Seaboard Coast Line and the Pullman-Standard Division of Pullman Incorporated, one a major railroad and the other a major railroad car building company, described standard 3-piece truck frames as oscillating with respect to the carbody in a parallelogram motion with displacement in the magnitude of one inch as being common throughout the test between carbody and truck sideframe. The centerplate bowl of the truck bolster oscillated with respect to the carbody in a translational as well as a swivel motion. Vertical and lateral accelerations of the two side frames were cyclical. Longitudinal accelerations were out of phase with respect to each other. Centerplate wear resulted in maximum centerplate longitudinal movement measured between carbody and truck bolster of 0.8 inches at 50 and 55 mph., indicating accelerated wear taking place during truck hunting.
In a conventional truck, in addition to the wheel axles, bearings and adapters, the side frames (which are the majority of the truck weight) are unsprung. The greater the weight of unsprung items the greater the magnitude of forces that are transferred into the car body for a given rail input. Friction damping in conventional trucks is created by two pieces of steel which rub together. Furthermore, the majority of the damping (when there are attempts made to solve dynamic problems) is obtained by hydraulic devices which are supplementary to the basic truck system. These hydraulic devices have the problem of being proportional to the velocity of motion rather than being proportional to distance travelled or rather than being proportional to the load in the car and, consequently, in a lightly loaded car where high frequency motion obtains, the damping becomes excessive and the forces transmitted to lading in the car became excessive.
Conventional trucks cannot adequately accommodate track twists found in service today and also maintain an adequate static load on the unloaded wheel. Without adequate loading, the unloaded wheel can easily climb the rail. Constant contact side bearings in conventional trucks further complicate the truck accommodation to the track twist problem because these side bearings require a certain amount of force to depress them.